1. Field of the Invention
The present invention relates to a heat treatment apparatus and a heat treatment method for heating a thin plate-like precision electronic substrate (hereinafter, simply referred to as “substrate”), such as a semiconductor wafer and a glass substrate for a liquid crystal display device, by irradiating the substrate with flashes of light.
2. Description of the Background Art
In a process for manufacturing a semiconductor device, impurity doping is an essential step for forming a pn junction in a semiconductor wafer. At present, it is common practice to implement the impurity doping through an ion implantation process and a subsequent annealing process. The ion implantation process is a technique of ionizing impurity elements such as boron (B), arsenic (As), and phosphorus (P) and causing ions of the impurity elements to collide against the semiconductor wafer with a high acceleration voltage, to thereby physically implant the impurities into the semiconductor wafer. The impurities thus implanted are activated through the annealing process. When a length of annealing time in this annealing process is equal to or longer than about several seconds, the implanted impurities are deeply diffused due to heat, and consequently a junction depth is excessively greater than required, which may hinder successful formation of a device.
Accordingly, in recent years, flash lamp annealing (FLA) is attracting attention, which is an annealing technique capable of heating a semiconductor wafer in an extremely short time. The flash lamp annealing is a heat treatment technique in which a xenon flash lamp (hereinafter, simply by the term “flash lamp”, a xenon flash lamp is meant) is used to irradiate a surface of a semiconductor wafer with flashes of light, to thereby cause only the surface of the semiconductor wafer implanted with impurities to rise the temperature in an extremely short time (several milliseconds or less).
A spectral distribution of emission from the xenon flash lamp ranges from ultraviolet to near-infrared regions. The wavelength of light emitted from the xenon flash lamp is shorter than that of light emitted from a conventional halogen lamp, and approximately coincides with a fundamental absorption band of a silicon semiconductor wafer. Thus, when a semiconductor wafer is irradiated with flashes of light emitted from the xenon flash lamp, the temperature of the semiconductor wafer can be raised rapidly, with only a small amount of light transmitted through the semiconductor wafer. Also, it has turned out that the irradiation of a semiconductor wafer with flashes of light in an extremely short time of several milliseconds or less allows the temperature to be selectively raised only in the vicinity of a surface of the semiconductor wafer. Therefore, when the temperature is raised in an extremely short time by means of the xenon flash lamp, only the activation of impurities can be achieved without causing deep diffusion of the impurities.
As a heat treatment apparatus using such a xenon flash lamp, US2006/0291835 discloses an apparatus in which a semiconductor wafer is placed on a hot plate and preheated to a predetermined temperature, and then irradiated with flashes of light emitted from flash lamps so that a surface temperature is raised to a desired processing temperature. Also, Japanese Patent Application Laid-Open No. 2009-231676 discloses an apparatus in which flash lamps are arranged on the upper side of a semiconductor wafer while halogen lamps are arranged on the lower side of the semiconductor wafer, so that the semiconductor wafer is preheated by being irradiated with light emitted from the halogen lamps and then a wafer surface is irradiated with flashes of light emitted from the flash lamps. In any case, the preheating is performed up to a temperature that does not cause diffusion of implanted impurities, and then the surface of the semiconductor wafer is irradiated with flashes of light so that the surface to a processing temperature is rapidly heated, to activate the impurities.
In the conventional flash lamp annealing described above, after the preheating, the surface of the semiconductor wafer is irradiated with flashes of light to thereby rapidly raise the surface temperature. In a case of performing the preheating with the hot plate as disclosed in US2006/0291835, a preheating time period of about one minute is required. Also in a case of performing the preheating with the halogen lamps as disclosed in Japanese Patent Application Laid-Open No. 2009-231676, a time length of several seconds to about ten seconds is required. Even though the flash heating is performed only for several milliseconds, the preheating requiring a long time not only lowers the throughput but also raises the temperature of component parts included a chamber that receives the semiconductor wafer therein. As a result, a problem arises that the rate of temperature drop of the semiconductor wafer after the flash heating is lowered.
Additionally, in a case where the surface of the preheated semiconductor wafer is irradiated with flashes of light to rapidly raise the surface temperature, the surface is a portion having the highest temperature with respect to the thickness direction of the semiconductor wafer, and inevitably the back surface is a portion having the lowest temperature with respect to the thickness direction of the semiconductor wafer. This consequently causes a tensile stress to act on the back surface, because large thermal expansion occurs in the surface of the semiconductor wafer while relative small degree of thermal expansion occurs in the back surface.
Particularly, there may be sometimes damage on the back surface of the semiconductor wafer. Such damage is often formed in a step prior to the flash lamp annealing. When the surface of the semiconductor wafer whose back surface has damage is irradiated with flashes of light and heated, a tensile stress acts on the back surface. This causes a problem that wafer cracking originating at the damage of the back surface occurs.
Moreover, normally, a device pattern is formed on the surface of the semiconductor wafer, and the light absorption rate varies depending on the pattern. Therefore, the absorption rate in a plane of the surface is not uniform. Accordingly, when the surface of the semiconductor wafer is irradiated with flashes of light, the temperature of a region where a pattern is formed and therefore the absorption rate is relatively high is higher than the temperature of a surrounding region. Thus, a variation in the temperature distribution occurs. That is, since the light absorption rate in the surface of the semiconductor wafer depends on the pattern, a problem arises that the temperature distribution in a wafer plane is non-uniform during irradiation with flashes of light. Such non-uniformity of the temperature distribution in a plane is more significantly observed as the intensity of irradiation with the flashes of light increases, and causes a variation in the degree of activation of impurities (in a region having a higher temperature, the activation progresses so that the sheet resistance value is lowered).